code: def test_generate_mixes_using_all_examples(self):
      sourceid_to_exids = {
          "source1": ["a", "b", "c", "d"],
          "source2": ["a", "b", "c", "d"],
      }
      rs = np.random.RandomState(seed=0)
      mixes = create_dataset_lib.generate_mixes_using_all_examples(
          sourceid_to_exids, rs
      )
      self.assertEqual(
          set(["a", "b", "c", "d"]), set(itertools.chain(*mixes))
      )
  
file_path: magenta/magenta/models/onsets_frames_transcription/create_dataset_lib_test.py
imports: import itertools
  from magenta.models.onsets_frames_transcription import create_dataset_lib
  import numpy
  import tensorflow.compat.v1
label_desc: Use image node_id 4 for calling the CreateDatasetLibTest obj's underlying member method code with example usage: obj.test_generate_mixes_using_all_examples() without return types